This invention relates generally to femoral head-neck prostheses and methods for their implantation.
Total hip replacement became a clinical reality for the first time in November, 1963, because of "cement fixation" of the components. The femoral head and neck were removed, the upper marrow canal of the femur was cleaned out (i.e., marrow contents removed), acrylic cement was poured into the marrow canal of the femur, and the metal femoral component was inserted into the liquid cement. In 10 to 15 minutes, the acrylic (methylmethacrylate) cement hardened and provided fixation for the femoral stem. The acrylic cement is similar to the acrylic dentists use to make dentures. Today, cement fixation of femoral components is still the most common means of fixation of the implant to the bone.
Cement fixation is the ultimate in form-filling contact with the bone. The liquid cement touches the entire inner surface of the upper femur. This type of fixation is generally successful for the short term (ten years), however in the long run, deterioration of the bone occurs and the cement and femoral component may loosen. Bone loss is caused by the cement and implant splinting the upper femur, preventing the upper femur from being subjected to natural bending. This is particularly a problem with younger patients (i.e., less than 50 years old).
Non-cemented or "press-fit" femoral components basically try to do the same thing as cemented implants; achieve solid fixation between the implant and the bone by maximally filling the medullary canal with the metal implant. In other words, the thinking is that the more closely and completely the metal implant fills the medullary canal, the better the fixation will be, and the more successful the result will be. However, experience shows this is not always the case.
The non-cemented femoral stems are larger and thicker than their cemented counterparts because the more flexible layer of acrylic cement is replaced with metal. Because a non-cemented stem is made of the same material and has a greater diameter than the cemented stem, it is stiffer. The greater the stiffness, the worse the splinting of the upper femur from the normal bending deflection that occurs in walking (strain). Although acceptable clinical results are achieved with non-cemented intramedullary femoral stems, non-cemented stems enerally have a more rapid rate of bone loss in the upper femur due to strain deprivation or what is commonly but incorrectly referred to as "stress shielding."
In summary, the fixation of all conventional intramedullary total hip femoral components depends on maximally filling the upper femur and the medullary canal with either cement and metal or metal alone. Not coincidentally, bone loss occurs with all of these implants.